Field of the Invention
The present invention relates to a device and a method for low-temperature liquefaction/cooling.
The invention may notably be concerned with a liquefaction method and device and a refrigeration method and device operating on helium.
The invention relates more specifically to a device for the low-temperature liquefaction and/or cooling of a working fluid comprising helium or consisting of pure helium, the device comprising a working circuit provided with a compression station and with a cold box, the compression station comprising one or more compression stages each using one or more compressors comprising a compressor impeller secured to a spindle mounted so that it can rotate on bearings, the spindle of each compressor being rotationally driven by an output shaft of a motor via a gear mechanism housed in a mechanical gearbox containing lubricating oil.
Related Art
Refrigerators or liquefiers operating at low temperature (for example below 80K, notably between 80K and 4K) conventionally use a working fluid (for example helium or a mixture containing helium) subjected to a working cycle that comprises a compression, possibly an expansion, a cooling, a heating. These devices in general require several stages for compressing the working gas. Each compression stage uses one or more compressor impellers. For example, these devices use compressors of the centrifugal type.
The compression stations that compress helium (or mixture containing helium) require a large number of compression stages in order to compensate for the low compression ratio of each stage (cf. for example FR2919716A1).
In particular, each compressor shaft must also rotate at a relatively high speed in order to ensure good compression efficiency.
The overall efficiency of a multistage compressor is dependent on a number of criteria and notably on:                the efficiency of the compression impellers and volutes (to increase the compression ratio),        the efficiency of the inter-stage cooling (to limit the pressure drop across the system),        the mechanical losses (notably caused by friction between moving parts).        
Leaks of working gas in the compression station at the interface between the rotating parts and the fixed parts are unavoidable. Particularly when the working gas is helium, relatively significant leaks of gas occur at the bearings supporting the shafts of the compressor impellers. In order to limit this loss of relatively expensive working gas, it is known practice to limit the leak at each bearing of each compression stage using components such as packings that form labyrinths against the gas, oil seals, floating ring seals, gas seals, etc.
Aside from the fact that these devices increase the cost of the installation, these known systems are not always suited to cooler/liquefier technology.
In addition, the oil present in the mechanism of the compression station must not contaminate the working gas (by mixing with the helium or by bringing in moisture and/or light hydrocarbons). Specifically, such impurities introduced into the working circuit carry the risk of causing plugging at cryogenic temperatures and of breaking the equipment.
Thus, such devices may optionally comprise recuperation systems for recovering potential leaks of helium as described for example in document FR2953913A1. According to that document, a buffer gas may be injected to collect the helium leaks.
The fluid tightness of a helium centrifugal compressor therefore employs technology that involves isolating the parts containing the lubricating oil by using a buffer gas (nitrogen). Buffering with gaseous helium is planned in order to avoid any leak of nitrogen into the working gas (helium).
This technology offers advantages but is the source of significant cost because this sealing system needs to be installed on each stage of the compressor.